The ratio of 2nd to 4th digit length: a proxy for testosterone, and susceptibility to HIV and AIDS?

The incidence of HIV and AIDS is high in sub-Saharan Africa and in male homosexuals. It is suggested that testosterone-mediated immune dysfunction may account for this pattern. The ratio of the length of the 2nd and 4th digit (2D:4D) is negatively related to prenatal and adult testosterone concentrations. There is evidence that black South Africans have lower 2D:4D ratios than most other populations and male homosexuals have lower 2D:4D ratios than male heterosexuals. Men with low 2D:4D ratios may also be more sexually active and/or more fertile than men with high ratio. We suggest that men and women with low 2D:4D are susceptible to HIV infection and AIDS and babies with low 2D:4D ratio susceptible to vertical transmission. Infection rates may therefore be reduced by education and condom supply directed towards low 2D:4D adults.